1. Field of the Invention
The invention relates to a unit for measuring absorbance using a microchip to execute solution analysis by an absorption spectrum process.
2. Description of Related Art
Recently an analysis method using a microchip called μ-TAS or Lab-on-a-chip has been considered in which, using precision processing technology of semiconductors and a technology for producing micromachines, chemical analyses and the like are performed in a more precise manner as compared to conventional devices. In the case of using μ-TAS for medical fields there are the following advantages:
(1) By reducing the amount of sample, such as, for example, blood, the burden on the patient can be reduced.
(2) By reducing the amount of reagent, the study costs can be reduced.
(3) Since the device is small, the study can be easily carried out.
By analysis using the absorption spectrum process with a microchip, the concentration of a desired enzyme which is contained in the blood plasma can be measured by the series of operations described below.
(1) Blood which was taken using a painless needle is delivered into the chip.
(2) The blood in the chip undergoes centrifugal treatment and is divided into plasma and hematocytes.
(3) The plasma and reagent are uniformly mixed with one another using a mixer and a mixture is produced therefrom.
(4) The mixture is delivered by means of a suction pump into a chamber for measuring absorbance.
(5) The mixture which was delivered into the chamber for measuring absorbance is irradiated with light from a light source and the attenuation of the light at a certain wavelength is measured. This effort is the measurement of absorbance.
The method of analyzing the concentration of an enzyme which is contained in the blood, such as, for example, GTP (glutamyl trans-peptidase), γ-GTP or the like, and which is needed, for example, to diagnose liver function, is disclosed in Japanese patent disclosure document JP-2004-109099 A.
This publication shows a process in which light which is emitted from a light source, such as a light emitting diode or the like and which is incident from the top of the chip, which is totally reflected in an extremely small channel in the chip which is filled with an analysis sample, such as, for example plasma, and which emerges from the top of the chip, is measured with a detector such as a silicon photodiode or the like.
The light emitted by the light emitting diode is, however, scattered light. It is extremely difficult to subject the light incident in the chip to total reflection overall in an extremely small channel. Therefore, there is the disadvantage that the absorbance cannot be measured with high precision. The measurement of absorbance by the arrangement described in the aforementioned publication and the light source as well as of the detector on the top of the microchip causes the occurrence of measurement errors; this is not desirable.
On the other hand, as shown in FIG. 6, in Japanese patent disclosure document JP-2004-77305 A, a technique is described in which light from a light source can be incident from one side of a microchip, in which the light is absorbed by a sample, with which a channel 41 is filled, for determination in the microchip 40, and in which the fluorescence which emerges from the other side is determined. It can be imagined that, in a process for use of a channel for determination of a straight sample, the microchip measurement can be performed with high precision when absorbance is measured using blood as the sample.
In the case of using a microchip for measuring the concentration of a sample by an absorption spectrum process, the length of the optical path cannot be shortened because a certain amount of absorption is required. The surface of the light entry and exit areas of the part for measurement of absorbance is, for example, roughly 0.49 mm2, i.e., very small. Therefore, a very narrow cell is needed. As a result, for exact measurement of absorbance, it is necessary for light with high parallelism to be incident. This is because, with high parallelism of the light, the light which passes through the side of the chamber and which emerges to the outside from the chamber for measuring absorbance is reduced and that measurement errors due to faulty light are reduced. Here, “faulty light” is defined as the light which passes through the chip part, but not through the chamber for measuring absorbance, and enters a light detector.
A laser can be imagined as an ideal light source. The wavelengths necessary for chemical analyses are however diverse. For laser light which is monochromatic light, for each required wavelength, a respective laser is needed. Costs are high. Therefore, lasers are not well suited to this function. It can also be imagined that there is no laser which emits the required wavelength. Therefore, as the light source, a light source is advantageous with an arrangement in which a discharge lamp, such as a xenon lamp or the like, which emits light in a continuous wavelength range is combined with a wavelength selection means, such as a wavelength selection filter or the like.
However, since the discharge lamp has a large arc spot and cannot emit parallel light with high efficiency, a measure is required against faulty light which passes through an area outside of the chamber for measuring absorbance and enters a light receiving apparatus. This is because the faulty light influences the measured value.
As a concept for incidence of light with little parallelism in the chamber of the microchip to measure absorbance, there is a process in which the inside of the chamber for measuring absorbance is coated with a fluororesin or with aluminum and in which the light is routed to the output using total reflection. Furthermore, there is also a process in which, on the end face of the chamber used to measure absorbance, a silica glass material is cemented and the substrate is made opaque to the measurement wavelength. As the means for delivery into the chamber for measuring absorbance, there is a process in which an optical fiber is inserted into the hole of the chip and total reflection is used within a cell.
In the process in which the inside of the chamber used to measure absorbance is coated, the light which is obliquely incident on the end face of the chamber used to measure absorbance is totally reflected from the inside and travels to the light receiving apparatus. Therefore, the optical path becomes longer than the length of the chamber used to measure absorbance, by which the transmission factor becomes less than in practice. As a result, the correct transmission factor cannot be measured. In the process in which the part, besides the end face, is made opaque, the light is totally reflected which has struck the side of the chamber used to measure absorbance with a flatter angle than the critical angle of total reflection, by which the optical path becomes longer than the length of the chamber for measuring absorbance. In this way, the transmission factor becomes less than in practice; this causes measurement errors. In a process in which the light is delivered using an optical fiber, the light emerging from the tips of the fibers propagates, by which reflection occurs within the chamber that is used to measure absorbance and there is the danger errors occur in the measurement values.